Process of producing chromic acid by electrolysis.



UNITED STATES PATENT CFFICE.

MAX LE BLANQ, OF FRANKFORT-ON-TIIE-MAIN, AND HERMANN REISENEG- GER, OF HOCHST-ON-THE-MAIN, GERMANY, ASSIGNORS TO THE FARE- W'ERKE, VORMALS MEISTER, LUOIUS & BRUNING, OF H6CHST-ON-THE- MAIN, GERMANY.

PROCESS OF PRODUCING CHROMIC ACID BY ELECTROLYSIS.

SPECIFICATION formingpart of Letters Patent No. 630,612, dated August 8, 1899.

Application filed August 19, 1898- Serial No. 689,009. (No specimens.)

To all whmn it may concern:

Be it known that we, MAX LE BLANO, pro fessor of chemistry, residing at Frankfort-onthe-Main, and HERMANN REISENEGGER, doctor of philosophy, residing at Hochst-on-the Main, Germany, citizens of the Empire of Germany, have invented certain new and useful Improvements in the Production of Ghromic Acid by Electrolytic Oxidation of Solutions of Ohromium-Oxid Salts, of which the following is a specification.

The process employed on a large scale for regenerating chromic acid from solutions containing chromium-oxid salts is complicated and scarcely rational. As is known, the solutions containing for the most part chromium sulfate are mixed with lime, and the paste thus obtained, which consists of chromium oxid, an excess of quicklime, besides large quantities of calcium sulfate, is heated to blood-red with access of air. Ohromate of lime is thus formed, which in an aqueous solution of sodium sulfate changes into soluble sodium chromate and calcium sulfate. On removing the latter the solution of sodium chromate may be converted into chromic acid by addition of sulfuric acid and employed for oxidation. Not only is the quantity of chro mic acid obtained by this process small com-' pared with the quantity of chromium oxid employed, but also considerable quantities of sulfuric acid are lost and many other inconveniences attend the process, for the sodium chromate can only have a sufficient oxidizing action in an acid solution, in this case a sulfuric-acid solution, and the whole of the acid added forms by this process of regeneration a worthless prod uct-namely, refuse calcium sulfate.

It has been proposed to regenerate the chromic acid from the solutions containing chromium-oxid salts by means of the electric current. (Zet'tschr. f. Angew. Chem, 1893, p. 360; Hilfsbuch f. d. Elecirotechm'lc 'v. Grawinkel at Strecker, 4: Aufi, p. 506, 1895; British Patent 5,542 of 1886.) According to the first process the chromium oxid is dissolved in an excess of caustic-soda lye, and this solution is poured into the anode-compartment of a vessel provided with a diaphragm, while as cathode a fluidfor instance, caustic-soda lye-is employed. At the anode sodium chromate is formed by the current, while causticsoda lye is produced at the cathode. The so dium chromate thus formed can be changed into bichromatebyfurtherelectrolysis. Apart from the great consumption of energy, according to the description, (at least five volts at a current density of somewhat more than two hundred amperes to the square meter,) this method of oxidation is impractical, as sodium chromate is employed for oxidation in a sulfuric-acid solution, and the loss of sulfuric acid is in this process accompanied bya further loss of the expensive caustic-soda lye. In the second process the chromic solutions which originate from exhausted galvanic elements are the main source of supply. These solutions are placed in the anode-compartment of a cell provided with a diaphragm, while in the cathode-compartment is put an acid solualum with zinc oxid dissolved therein. If the electric current be conducted through it, then chromic acid or chromates are formed at the anode and metallic zinc separates at the cathode. It is at once evident that to carry out the process in this form on a large scale would in most cases be unsatisfactory, as the requisite quantities of zinc sulfate would. be wanting. But could this process not be carried out with advantage in employing a pure sulfuric-acid solution instead of the cathode solution, while having at the anode chromium sulfate in an aqueous solution of sulfuric acid to be regenerated? Suppose electrolysis taking place, then hydrogen 'tion of zinc sulfate ora solution of ammonia is developed at the cathode, while chromic tion changes take place. (Vz'de Le Blane, The

Elements of Electrochem'istry, (,hapt. IV, 1896, London, Macmillan & 00., Ltd.) \Ve have originally at the cathode as ions H, 1180,, S6,; at the anode, in a strong acid solution, also II, 1180,, S0 and Cr, negative ions of chromium sulfuric acid of the type Cr (S0 and CrO Apart from the slight action of the diffusion the positive ions move on the passing of the current to the cathode and the negative ions to the anode. Consequently S?) and HO pass from the cathode through the diaphragm to the anode and from the latter H and Or to the cathode. The re sult is that some chromium oxidthe quantity of Or not being great-4s formed at the cathode, and besides an increase of sulfuric acid has taken place at the anode and a decrease at the cathode. That this latter concentration change is of importance is shown by an experiment which we have made on another occasion, the result of which, however, may also be applied in this case. We had the same solution at the anode and cathode, containing in one-hundred grams thirty.- five grams ll SO and 7.5 grams 01- 0 After having oxidized one per cent. of the chromium oxid the quantity of sulfuric acid at the anode had increased about fifteen per cent., if eompared to the original quantity.

The above explanation shows that to carry out this process would cause great drawbacks. It would, for instance, be necessary to separate often the excess of sulfuric acid from the anode solution by means of lime, as the conductivity of the solution is only good within certain concentration changes of the sulfuric acid. By this separation (apart from the actual labor) besides the sulfuric acid also some chromium solution is lost, and the lime is liable to cause troublesome impurities. Moreover, the sulfuric acid becomes too diluted at the cathode and will have to be replaced by fresh concentrated acid, or else the electric current would encounter too much resistance. The dilute acid, with the chromium oxid migrated over, will have to beconsidered as lost.

All the drawbacks mentioned are avoided by a new rational process of great economical importance, as has been shown by experiments on a large scale. There is used the solution of a chromium salt in the corresponding acid-for instance, chromium sulfate dissolved in sulfuric acid-and preferably a so lution of the highest possible electric conductivity, about one hundred grams C13 0 and three hundred and fifty grams I-I SO can be made up with water to one liter. This solution is placed in the anode and cathode compartments of a vessel coated with lead, provided with adiaphragm, and having lead plates as electrodes. By the action of the current chromic acid is formed at the anode, while hydrogen is evolved at the cathode. As already said above, a migration of sulfuric acid also takes place, the concentration increasing at the anode and decreasing at the cathode. The solution oxidized at the anode can at once be employed in manufacture,and the chromic acid therein being thus retransformed into chromium oxid can then be moved to the cathode,while the solution atfirst in the cathode-compartment is now transferred to the anode-compartment. The cathode solution is at the beginning of the second operation richer in sulfuric acid than the anode solution, yet on the, passing of the current the excess of sulfuric acid is transferred to the anode. By this arrangement of circulation, moving alternately the solution from the cathode to the anode, it results that an accumulation of sulfuric acid nowhere takes place and that, in fact, the same solution can be employed indefinitely in its unchanged composition and without any loss in substance as an excellent carrier of oxygen. At a current density of more than three hundred amperes per square meter the tension amounts to hardly 3.5 volts at about 50 centigrade.

Having thus described our invention, we claim as new and desire to secure by Letters Patent The herein-described process of producing chromic acid, which consists in first electrolyzing a solution of a chromium-oxid salt in an aqueous solution of the corresponding acid, as for instance solution of chromium sulfate in aqueous sulfuric acid, then employing the mixture of chromic and other acid resulting from the electrolysis for reactions by which it is reconverted into the solution of the oxid salt in an acid, charging this at the cathode when the fluid formerly at the cathode has been transferred to the anode for formation again of chromic acid, and so oncontinuously, substantially as set forth.

In testimony that we claim the foregoing as our invention we have signed our names in presence of two subscribing witnesses.

MAX LE BLANO. I-IERMANN REISENEGGE Pt.

\Vitnesses:

BERNHARD LEYDECKER, ALFRED BRISBOIS. 

